Circuit arrangement for switching a current of and off without the occurrence of overcurrent

ABSTRACT

There is provided a circuit arrangement for switching a current on and off without overcurrent. The circuit arrangement includes a current source, a load associated with the current source, a switching transistor for switching the current source on and off, where the switching transistor has a parasitic capacitance between a control electrode and a first output electrode, and a shorting device between the control electrode and the first output electrode of the switching transistor for switching off the current source. The current can be used, for example, to drive a laser diode.

FIELD OF THE INVENTION

The invention relates to a circuit arrangement for switching a currenton and off without overcurrent, that current being used to drive a laserdiode, for example.

BACKGROUND OF THE INVENTION

Circuit arrangements for switching a current driving a load on and offare known.

FIG. 1 shows such a circuit arrangement, which uses a transistor 20′ asa current source or current sink. The control electrode of thetransistor 20′ is connected to a DC voltage source 80′. The currentsource 20′ is connected in series with a load 40′, for example with alaser diode. A switching transistor 50′, which is likewise connected inseries with the current source 20′, is used as a switch for the currentsource. The switching transistor 50′, which is in the form of afield-effect transistor, for example, is connected by means of its gateelectrode 51′ to a driving device which comprises the transistors 75′and 70′, for example. The input 1 of the driving device 70′, 75′ has aswitch-on/switch-off signal P applied to it, as illustrated in FIG. 2,for example. Between the gate 51′ and the drain electrode 52′ of theswitching transistor 50′ there is a manufacture-dependent parasiticcapacitance 60′, which is also known as the Miller capacitance. As willbe explained later, this parasitic capacitance is the cause ofovercurrents that are brought about when the current source 20′ isswitched off. The gate/source voltage GS of the switching transistor 50′controls the state of the switching transistor. If this gate/sourcevoltage GS is lower than the threshold voltage of the switchingtransistor 50′, the current IL is interrupted by the switchingtransistor 50′. The gate/source voltage GS is S controlled by means ofthe driving device 70′, 75′. However, when the switching transistor 50′and hence the current source 20′ are disconnected, an overcurrent, alsocalled a current spike, is produced which has its origins in theparasitic capacitance 60′ of the switching transistor 50′. This isbecause a falling edge of the gate/source voltage GS on the switchingtransistor 50′ (the profile of said gate/source voltage being shown inFIG. 4) causes the parasitic capacitance 60′ to transfer the suddenvoltage change on the control electrode 51′ to the output electrode 52′of the switching transistor 50′ and to drive the transistor 20′ at ahigher level. This causes an overcurrent through the load 40′, as shownin FIG. 3 at the time t_(a). The overcurrent flows until the parasiticcapacitance's charge has been reversed. The time t_(a) characterizes thetime at which the switch-off signal is applied to the input 1 of thedriving device 70′, 75′. In the case of certain applications, suchcurrent spikes can result in the load 40′ being destroyed, as can be thecase with a laser diode, for example.

SUMMARY OF THE INVENTION

The invention is now based on the object of improving the initiallydescribed circuit arrangement for switching a current on and off suchthat overcurrents can be avoided when a current source is switched off.

The invention solves this technical problem with the features of claim1.

Accordingly, a circuit arrangement for switching a current on and offwithout overcurrent is provided. The circuit arrangement has a currentsource, a load associated with the current source and a switchingtransistor for switching the current source on and off. Depending onmanufacture, the switching transistor contains a parasitic capacitancebetween the control electrode and a first output electrode, which causescharge-reversal currents when the current source is switched off. Ashorting device, connected between the control electrode and the firstoutput electrode of the switching device, for switching off the currentsource can be used to prevent the charge-reversal currents fromresulting in overcurrents in the load.

Expediently, the shorting device is in the form of a transistor whosecontrol electrode can have a switch-on/switch-off signal applied to it.One output electrode of the shorting device is connected to the firstoutput electrode of the switching transistor, and the other outputelectrode of the shorting device is connected to the control electrodeof the switching transistor. In this way, the shorting device shorts theswitching transistor and hence switchs off the current source.

In order to be able to shorten the input and output cycles, a pull-updevice is connected in parallel with the shorting device and ensuresthat the potential on the switching transistor's control electrode canbe raised at the switch-on time.

The current source, the switching transistor, the shorting device and/orthe pull-up device can be in the form of bipolar transistors,field-effect transistors, particularly MOS transistors, or combinationsof these technologies.

The circuit arrangement is suitable particularly for driving a laserdiode as a load.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to anexemplary embodiment in conjunction with the appended drawings, inwhich:

FIG. 1 shows a circuit arrangement for switching a current on/off inaccordance with the prior art,

FIG. 2 shows the time-dependency diagram for a switch-on/switch-offsignal,

FIG. 3 shows the load current IL through the load 40′,

FIG. 4 shows the time-dependency diagram of the potential profile on thecontrol electrode of the switching transistor shown in FIG. 1,

FIG. 5 shows a circuit arrangement in accordance with the invention,

FIG. 6 shows the time-dependency diagram for a switch-on/switch-offsignal in accordance with FIG. 2,

FIG. 7 shows the time-dependency diagram of potential profile on thecontrol electrode of the switching transistor shown in FIG. 5,

FIG. 8 shows the time-dependency diagram of current through theparasitic capacitance shown in FIG. 5, and

FIG. 9 shows the time-dependency diagram for a load current withoutovercurrent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 5 shows a circuit arrangement 10 that contains components of thecircuit arrangement shown in FIG. 1. A load, for example a laser diode40, is connected to the drain electrode of an exemplary n-channelfield-effect transistor 20 operated as a current source. The gate of thecurrent source 20 has a constant voltage applied to it that is deliveredby the DC voltage source 80. The source electrode of the current source20 is connected to the drain electrode 52 of an exemplary n-channelfield-effect transistor acting as a switching transistor 50, so that theload 40, the current source 20 and the switching transistor 50 form aseries circuit. The source electrode 53 of the switching transistor 50is connected to ground. In addition, there is a shorting transistor 30,for example in the form of an n-channel field-effect transistor, whosedrain electrode 33 is connected to the gate 51 of the switchingtransistor 50, whereas the source electrode 32 of the shortingtransistor 30 is connected directly to the drain electrode 52 of theswitching transistor.

It is important to point out that the shorting transistor 30, unlike thecircuit arrangement shown in FIG. 1, connects the gate 51 of theswitching transistor 50 in the conducting state not to ground but to thedrain electrode 52 of the switching transistor 50. In this way, as willbe explained in detail later, the manufacture-dependent parasiticcapacitance 60 between the gate 51 and the drain electrode 52 of theswitching transistor 50 can be shorted at the time at which theswitching transistor 50 is switched off, which simultaneously connectsthe switching transistor 50 to a two-terminal network across which thethreshold voltage drops.

The gate 31 of the shorting transistor 30 is connected to an inputconnection 1 to which the switch-on/switch-off signal P shown in FIG. 6can be applied. The shorting transistor 30 can have a pull-up element 70connected in parallel with it, for example a pull-up transistor in theform of a p-channel field-effect transistor whose gate is likewiseconnected to the input 1 of the circuit arrangement. Unlike the shortingtransistor 30, the pull-up transistor 70 is in the form of a p-channelfield-effect transistor. The source electrode of the pull-up transistor70 is connected to the operating voltage VDD, whereas the drainelectrode is connected to the gate 51 of the switching transistor 50. Inthis way, the gate 51 of the switching transistor 50 can be pulled morequickly at the switch-on time to a potential which is above thethreshold voltage of the switching transistor 50, which means that theswitching transistor can switch on more quickly and hence can switch onthe current source 20 more quickly.

A detailed explanation will now be given, in conjunction with FIG. 6 to9, of the way in which the circuit arrangement 10 works when the currentis switched off.

Consideration will first be given to the phase during which theswitching transistor 50 and hence the current source 20 are on. Duringthis time, the switch-on signal P shown in FIG. 6 is applied to theinput 1 and adopts a low level during this time in the elucidatedexample. On account of the low level, the shorting transistor 30 switchsoff and the potential on the gate 51 of the switching transistor 50 isheld at operating voltage by means of the pull-up transistor 70, whichis on during this period, as shown in FIG. 7. Since the operatingvoltage is chosen to be higher than the threshold voltage of theswitching transistor 50, a current consequently flows through theswitching transistor 50 and the load current IL flows through the load40 and the current source 20, as shown in FIG. 9. In other words, thecurrent source 20 is on.

It will now be assumed that a switch-off signal P is applied to theinput 1 of the circuit arrangement at the time t₁, so that the potentialat the input 1 changes from low to high. This time t₁ is shown in FIG.6.

At this time, the shorting transistor 30 starts to conduct and connectsthe gate 51 of the switching transistor 50 to the drain electrode 52.Consequently, the charge-reversal current I2 in the parasiticcapacitance 60 flows via the shorting transistor 30, as shown in FIG. 8.This charge-reversal current thus no longer flows as a current spikethrough the load 40. The current I1 flowing via the pull-up transistor70 drains to ground essentially through the shorting transistor 30 andthen via the switching transistor 50. This phase lasts until the timet₂, at which the voltage on the gate 51 of the switching transistor 50has reached the latter's threshold voltage Vth. This phase is shown inFIG. 7 by the falling edge of the gate/source voltage between the timest₁ and t₂. As soon as the voltage on the drain electrode 52 of theswitching transistor 50 has become high enough to achieve a dip belowthe threshold voltage Vth of the current source 20, the current source20 is switched off. Since the shorting transistor 30 shorts the gate 51and the drain electrode 52, the same potential is applied to bothpoints. Consequently, as mentioned, the charge-reversal current in theparasitic capacitance 60 can flow via the shorting transistor 30. Theload current IL, which is free from overcurrent, shown in FIG. 9 thusfalls to almost zero between the times t₁ and t₂. The condition for theload current IL to be disconnected by the switching transistor 50 isthat the constant voltage applied to the gate of the current source 20is lower than the sum of the threshold voltages of the current source 20and the switching transistor 50.

To be able to speed up the input and output switching processes furtheror to be able to extend the operating range, the potentials on the gateof the current source or on the gate of the switching transistor 50 canbe lowered when switching off or raised when switching on by furtherpermanently connected or dynamically connectable current sinks.

List of References

-   1 Input-   10 Circuit arrangement-   20, 20° Current source-   30 Shorting device-   31 Gate-   32 Source electrode-   33 Drain electrode-   40, 40′ Load, laser diode-   50, 50′ Switching transistor-   51, 51° Control electrode, gate-   52, 52′ Drain electrode-   53, 53′ Source electrode-   60, 60′ Parasitic capacitance-   70′, 75′ Driving device-   70 Pull-up transistor-   80, 80′ DC voltage source-   VDD Operating voltage-   IL Load current-   GS Gate/source voltage-   P Switch-on/switch-off signal

1. A circuit arrangements for switching a current on and off withoutovercurrent, comprising: a current source; a load associated with thecurrent source; a switching transistor for switching the current sourceon and off, wherein the switching transistor has a parasitic capacitancebetween a control electrode and a first output electrode, which is notat constant potential; and a shorting device between the controlelectrode and the first output electrode of the switching transistor forswitching off the current source.
 2. The circuit arrangement as claimedin claim 1, wherein the shorting device comprises a transistor having: acontrol electrode for having a switch-on/switch-off signal applied toit; an output electrode connected to the first output electrode; and anoutput electrode connected to the control electrode of the switchingtransistor.
 3. The circuit arrangement as claimed in claim 1, whereinthe shorting device has an associated pull-up device.
 4. The circuitarrangement as claimed in claim 1, wherein one or more of the currentsource, the switching transistor, the shorting device and the pull-updevice are in a form selected from the group consisting of a bipolartransistor and a field-effect transistor.
 5. The circuit arrangement asclaimed in claim 1, wherein the load is a laser diode.
 6. The circuitarrangement as claimed in claim 1, wherein the current is switched offby the switching transistor if a constant voltage applied to a controlelectrode of the current source is lower than a sum of thresholdvoltages of the current source and the switching transistor.